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Electron Transport in Integrated Quantum Systems

Yan, C; (2016) Electron Transport in Integrated Quantum Systems. Doctoral thesis , UCL (University College London). Green open access

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In this thesis, integrated quantum devices defined using a split gate technique are studied experimentally. These integrated devices provide a novel platform to investigate the property of quantum systems, such as spin polarization, via non-local measurement. Information extracted from these integrated devices leads to a comprehensive understanding of the puzzling phenomenon such as the 0.7 anomaly. Meanwhile, these devices are possibly suitable for studying quantum entanglement because perturbation due to measurement is minimized in the non-local setup. Devices demonstrated here are also promising to be used as a building block such as quantum injector/detector or quantum bus (which is a information channel where quantum information can be transported coherently) for more complicated quantum systems. In the first experiment, a transverse electron focusing in n-type GaAs heterojunction is present where pronounced splitting of odd focusing peaks are observed. From the asymmetry of sub-peaks of the first focusing spin polarization is extracted directly, this provides direct evidence for intrinsic spin polarization in a quasi-one-dimensional system. Parameters which may affect transverse electron focusing are studied systemically. Changing the shape of the injector, thus tuning the adiabaticity of the injection process, can influence the presence of peak splitting or not, with the sharp (non-adiabatic) injector the peak splitting is absent while peak splitting is observed with the flat (adiabatic) injector. Adjusting the length of injector affects the spin polarization, the longer the channel the higher the spin polarization can be achieved. This highlights the role of exchange interaction which results in the spin polarization in the quasi-1D channel. Applying a dc source-drain bias leads to such a result, peak splitting is preserved with negative bias while it smears out with positive bias when the bias is above a particular value (0.5 mV in the experiment), this proves the existence of spin-gap. In the second experiment, the coupling between different quantum devices are investigated by using an integrated quantum device consisting of an QPC and electronic cavity, where the cavity is defined with the arc-shaped gate and an inclined reflector. Unique features such as the double-peak structure occurs in the 1D-2D transition regime of the arc-QPC and 5 fine oscillations associated with conductance plateaus and 0.7 anomaly are observed when the reflector voltage is sufficiently negative and these features smear out when the reflector voltage is less negative. The double-peak structure and fine oscillations are proved to arise from the coupling between the discrete states in the QPC and continuum cavity state by the manifestation of Fano resonance via tuning reflector voltage or small transverse magnetic field. In the third experiment, quantum interference in a double-cavity system is studied by magneoresistance measurement. An unique evolution of the line shape of the magnetoresistance are observed, the magnetoresistance has a Lorentzian shape, corresponding to ergodic and chaotic motion, when the injector conductance is sufficiently small and then alters into linear line shape arising from non-ergodic and regular motion when injector is opens a bit more and finally a Lorentzian shape when the injector opens even further. Apart from the line shape, the strength of the magnetoresistance is found to fluctuate with injector conductance, it is enhanced at conductance plateaus and weakens elsewhere. Such behaviours are likely to arise from both deformation of the arc-shaped potential barrier at the vicinity of injector and detector QPC as well as the non-uniform spatial distribution of the cavity state.

Type: Thesis (Doctoral)
Title: Electron Transport in Integrated Quantum Systems
Event: University College London
Open access status: An open access version is available from UCL Discovery
Language: English
UCL classification: UCL
UCL > Provost and Vice Provost Offices
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science > Dept of Electronic and Electrical Eng
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
URI: https://discovery.ucl.ac.uk/id/eprint/1531982
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